System and method for short-distance wireless communications, and applications thereof

ABSTRACT

A first portable wireless communication device enters a PICONET and establishes unencrypted communications with a second wireless communication device, the PICONET master. The first portable wireless communication device transmits identification information to the second wireless communication device using a conventional short-distance communications protocol. The second wireless communication device transmits, in a novel FEMTONET communications mode, an encryption key to the first portable wireless communication device. The first portable wireless communication device then securely transmits encrypted messages based on the encryption key to the second wireless communication device using the conventional short-distance communications protocol.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/874,794, filed Jun. 24, 2004, which claims the benefit of U.S.Provisional Application No. 60/482,016, filed Jun. 25, 2003, each ofwhich is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to wireless communications. Moreparticularly, it relates to securing communications with wirelessdevices over a short distance.

BACKGROUND OF THE INVENTION

The Institute of Electrical and Electronics Engineers (IEEE) haspromulgated standards designated 802.11 and 802.15 for short-distancewireless networks. These standards describe protocols and proceduresthat allow wireless devices in a Personal Operating Space (WLAN,PICONET, PAN) or IEEE Personal Area Network to communicate.

Generally speaking, a short-distance wireless device designed to operateas part of a PICONET, WLAN or PAN has a transmission radius of at least10 meters. This is based on, for example, a short-distance wirelessdevice having a 2.4 GHz, 1 milli-watt transmitter operating at about 1Mb/s. Multiple short-distance wireless devices having peer-to-peercommunications capabilities can operate in overlapping PICONETs or PANsto form part of a larger communications network (termed a Scatter Net).

The general trend in short-distance wireless communications has been toincrease the operating range of conventional PICONETs and PANs. Forexample, more recent PICONET and PAN standards call for a 30-meteroperating range. This increase in range is disadvantageous forshort-distance wireless applications where security is important. Forexample, by increasing the transmission range of short-distance wirelessdevices in a PICONET or PAN, it becomes more difficult to limit thenumber of wireless devices that receive network messages, and as aresult, it becomes more likely that network message will be received andeither erroneously acted upon by an unintended recipient or perhaps evenhacked by an unauthorized recipient. Encrypting network messages canovercome the issues associated with increased operating ranges, butusing encryption makes it difficult for wireless devices to seamlesslyenter and exit a conventional PICONET or PAN.

What is needed are new, more secure systems and methods forshort-distance wireless communications that overcome the limitations ofa conventional PICONET or PAN. Specifically what is needed is a methodof exchanging session keys between a user entering the PICONET and atrusted body supervising the physical location.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a system and method for secure,short-distance wireless communications. In an embodiment, the presentinvention provides a system that is compliant and compatible with theestablished conventional communication systems and adds a method wherebya proprietor of a physical space can implement, for example, a node(proprietor node) in a PICONET that can securely send session encryptionkeys or session keys over the PICONET to a particular portable wirelesscommunication device. The transmission of the session key is onlyreceivable in a small physical volume in the PICONET (approximately 1cubic foot) which is referred to herein as a FEMTONET, which surroundsthe exact physical location of the portable wireless communicationdevice. As it is very unlikely that two portable wireless communicationdevices can occupy the same FEMTONET without being noticed, the presentinvention provides a convenient secure method of passing session keys toonly one party. The session keys allow the receiving portable wirelesscommunication device or party to encrypt its communications with theproprietor node in order to prevent other nodes in the PICONET fromreceiving data that may be confidential.

In an embodiment, communications are established between a firstportable wireless communication device or node and the proprietorwireless communication device or node. This typically occurs, forexample, when the first wireless communication device enters within theoperating range of a PICONET of which the proprietor wirelesscommunication device is the master. This may involve the first wirelesscommunication device entering into a communications protocol (in somestandards called a “Discovery Protocol”) with the second wirelesscommunication device so that the first wireless communication device canbecome a node of the PICONET.

The communications protocol exchange between the first and the secondwireless communication devices are conducted in a PICONET communicationsmode. As used herein, PICONET communications mode is defined as anyconventional short-distance communications mode such as, for example,WI-FI, IEEE 802.11, BLUETOOTH, or IEEE 802.15 compliant communicationsmode.

After communications between the first and the second (e.g., proprietor)wireless communication devices have been established, the first wirelesscommunication device sends, in PICONET communications mode,identification information to the second wireless communication device.In an embodiment, this identification information includes a universallyunique identification number associated with the first wirelesscommunication device. The first communication device is now a node inthe PICONET and able to communicate and utilize the resources providedby the other nodes of the PICONET.

In an embodiment, such as a shop or secure building, the first wirelesscommunication device enters the PICONET at an entrance. This entrance isequipped with at least one antenna and radio frequency reflectorssuitable for reflecting the RF signals from the first wirelesscommunication device so that the location of the first wirelesscommunication device is encrypted by the reflections to allow atransmission from the proprietor node to concentrate the in phase signalinto a FEMTONET leaving the majority of the PICONET with a signal thatis out of phase and that is interpreted by other wireless communicationdevices as a collision and ignored. This message sent to the FEMTONET isused to download to the first wireless communication device a sessionkey for encrypting data to be sent back to the proprietor.

After receiving the session encryption key, the first wirelesscommunication device transmits, in PICONET communications mode,encrypted information based on the encryption key to the proprietorwireless communication device. This information can only be decoded bythe proprietor wireless communication device. Thus, the informationtransmitted by the first wireless communication device to the secondwireless communication device is secure even though the information isbeing transmitted in PICONET communications mode.

In an embodiment of the present invention, the second wirelesscommunication device transmits the session encryption key to the firstwireless communication device via a plurality of signals thatconstructively interfere in the vicinity of a spatial locationdetermined by a locator of the second wireless communication device.This spatial location corresponds to the physical location of the firstwireless communication device. The signals transmitted by the secondwireless communication device destructively interfere in locations aboutthe spatial location determined by the locator.

In an embodiment, the locator of the second wireless communicationdevice carries out a time-reversal-mirror function. This functionenables the second wireless communication device to locate and track thelocation of the first wireless communication device.

In an embodiment, the shape of the spatial location determined by thelocator of the second wireless communication device is an approximatelyspherical volume having a diameter of less than three feet. In anotherembodiment, the diameter of the approximately spherical volume is lessthan one foot.

In an embodiment of the present invention, the plurality of signals sentby the second wireless communication device, when operating in FEMTONETcommunications mode, are transmitted by a single antenna coupled to thesecond wireless communication device. At least one of the signalsarrives at the spatial location determined by the locator after beingreflected by a reflector.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 is a diagram of an example PICONET.

FIG. 2 is a diagram illustrating a wireless communication deviceattempting to enter the PICONET of FIG. 1.

FIG. 3 is a diagram illustrating the PICONET of FIG. 1 after it has beenexpanded to include an additional wireless communication device.

FIG. 4 is a diagram illustrating the use of radiofrequency (RF)reflectors for securing short-distance wireless communications.

FIG. 5A is diagram illustrating the use of multiple antennas forsecuring short-distance wireless communications.

FIG. 5B is a diagram illustrating an example of secure short-distancewireless communications.

FIG. 6 is a flowchart illustrating the steps of a method forestablishing secure short-distance communications.

FIG. 7 is a diagram illustrating an example application of the method ofFIG. 6.

DETAILED DESCRIPTION OF THE INVENTION Overview of the Invention

The present invention relates generally to systems and methods forshort-distance wireless communications, and applications thereof. In anembodiment of the present invention, a first portable wirelesscommunication device enters a PICONET and establishes unencryptedcommunications with a second wireless communication device, the PICONETmaster. The first portable wireless communication device transmitsidentification information to the second wireless communication deviceusing a conventional short-distance communications protocol. The secondwireless communication device transmits, in a novel FEMTONETcommunications mode, an encryption key to the first portable wirelesscommunication device. The first portable wireless communication devicethen securely transmits encrypted messages based on the encryption keyto the second wireless communication device using the conventionalshort-distance communications protocol.

Example System Embodiment of the Present Invention

FIG. 1 illustrates an example PICONET 100. As shown in FIG. 1, PICONET100 includes a dual communications mode wireless communication device102 and three wireless communication devices 104, 106, and 108. Wirelesscommunication device 102 is the master of PICONET 100. The threewireless communication devices 104, 106, and 108 are slaves that adoptthe timing of wireless communication device 102 and respond to anymessages from wireless communication device 102. The outer edge ofPICONET 100 shown in FIG. 1 represents the maximum effectivetransmission range of wireless communication device 102. A fourthwireless communication device 110 shown in FIG. 1 is not yet a part ofPICONET 100.

As described in more detail below, wireless communication device 102 iscapable of communicating with any wireless communication device inPICONET 100 in a PICONET communications mode or a FEMTONETcommunications mode. As noted above, PICONET communications mode isdefined as any conventional short-distance communications mode such as,for example, a Wi-Fi, a BLUETOOTH compliant communications mode or anIEEE 82.11, or an IEEE 802.15 compliant communications mode. FEMTONETcommunications mode is defined as a novel short-distance communicationsmode wherein communications signals intended for reception by aparticular wireless communication device of PICONET 100 are sent bywireless communication device 102 in a manner that precludes otherwireless communication devices within the transmission range of wirelesscommunication device 102 from receiving or intercepting the signals. Inan embodiment, when operating in FEMTONET communications mode, thesignals sent by wireless communication device 102 constructivelyinterfere in the vicinity of a wireless communication device selected bywireless communication device 102 to receive the signals and ideallydestructively interfere in every other location in PICONET 100. Becausethe signals sent by wireless communication device 102, when operating inFEMTONET communications mode, destructively interfere in every locationof PICONET 100 except in the vicinity of the intended recipient wirelesscommunication device, other wireless communication devices of PICONET100 are precluded from receiving or intercepting the signals.

As illustrated in FIG. 1, each of the wireless communication devices104, 106, and 108 are in communications with wireless communicationdevice 102. Wireless communication device 104 communicates with dualcommunications mode wireless communication device 102 via acommunications channel 114. Wireless communication device 106communicates with dual communications mode wireless communication device102 via a communications channel 116. Wireless communication device 108communicates with dual communications mode wireless communication device102 via a communications channel 118. Although not shown, additionalcommunications channels can exist between the various wirelesscommunication devices that makeup PICONET 100.

Each of the three communications channels 114, 116, and 118 can be anystandard short-distance communications channel such as, for example, aBLUETOOTH compliant communications channel or an IEEE 802.11 compliantcommunications channel, and/or a FEMTONET communications channelaccording to the present invention. As will be understood by personsskilled in the relevant arts given the description herein, the threecommunications channels 114, 116, and 118 can be implemented by asingle, omni-directional broadcast by wireless communication device 102,when operating in a PICONET communications mode. When wirelesscommunication device 102 is operating in FEMTONET communications mode,only one of the three communications channels 114, 116, and 118 can beactive at any given time. As explained in more detail below, in anembodiment, the three communications channels 114, 116, and 118 carryencrypted messages.

FIG. 2 is a diagram illustrating wireless communication device 110attempting to enter PICONET 100. As shown in FIG. 2, wirelesscommunication device 110 is within the transmission range of wirelesscommunication device 102.

In order to enter PICONET 100, wireless communication device 110transmits a signal 202 indicating its presence and its desire to becomea communications node of PICONET 100. The signal 202 is capable of beingreceived by wireless communication devices 102, 104, and 108. As shownin FIG. 2, wireless communication device 106 is beyond the maximumeffective transmission range of wireless communication device 110.

As will be understood by persons skilled in the relevant art given thedescription herein, it is desirable in many applications that thecommunications established between two or more of the wirelesscommunication devices in PICONET 100 such as, for example, wirelesscommunication device 102 and wireless communication device 110 besecure. This security can be achieved if an encryption key is used toencrypt messages before they are transmitted. The problem that arises,however, when wireless communication device 110 tries to enter PICONET100 is how can an encryption key be passed between wirelesscommunication device 102 and wireless communication device 110 withoutthe encryption key being intercepted by another wireless device presentwithin the transmission range of either wireless communication device102 or wireless communication device 110. How this can be accomplishedis described in detail below with reference to FIGS. 4 through 6.

FIG. 3 is a diagram illustrating PICONET 100 after wirelesscommunication device 110 has successfully entered PICONET 100. As shownin FIG. 3, a secure communications channel 300 has been establishedbetween wireless communication device 102 and wireless communicationdevice 110.

Wireless communication device 110 encrypts messages to be sent towireless communication device 102 with an encryption key received fromwireless communication device 102. As noted above, how this encryptionkey is securely passed to wireless communication device 110 is describedin detail below with reference to FIGS. 4, 5, and 6. In an embodiment,messages received by wireless communication device 110 from wirelesscommunication device 102 are also encrypted. These messages can beencrypted with an encryption key provided by wireless communicationdevice 102 or with an encryption key provided by wireless communicationdevice 110. Once wireless communication device 110 has received anencryption key from wireless communication device 102, wirelesscommunication device 110 can send, in PICONET communications mode, anencryption key (encrypted using the encryption key received fromwireless communication device 102) to wireless communication device 102.Because only wireless communication devices 102 and 110 know theencryption key(s) used to encode messages passed via communicationschannel 300, other wireless communication devices within thetransmission ranges of wireless communication devices 102 and 110 cannotdecode these messages.

FIG. 4 illustrates a mobile communication device 110 transmitting itsdiscovery protocol packages in example PICONET 100 in which radiofrequency (RF) reflectors are used in accordance with teachings of thepresent invention. In this example embodiment of the present invention,there are several RF reflectors such as RF reflectors 111 and 112, whichmay be existing structures such as walls, or which may be deliberatelyengineered and placed to enhance RF reflections.

The signal transmitted by communication device 110 has several paths tothe wireless communication device 102 (proprietor node). These pathsare, for example, either direct or via the RF reflectors 111 and 112.The signal received at the wireless communication device 102 is the sumof, in this case, the signal directly from communication device 110 andthe reflections from the passive RF reflections 111 and 112. This summedreceived signal may be considered to have some properties or signatureof the environment of the PICONET 100. If the reflection devices such asRF reflectors 111 and 112 are not extensively large, as defined by thepercentage of the subtended solid angle to the wireless communicationdevice 110, the direct signal will be larger and the communication canbe correctly interpreted. The reflected signals would normally betreated as background noise.

However, provided that the RF reflectors 111 and 112 are large withrespect to the wavelength (50-100 cm in any direction) the reflectionswould contain a signal that is coherent with the direct signal anddelayed with respect to the direct signal. In the case of PICONETs basedon, for example, the above referred to IEEE 802.11 and IEEE 802.15standards and the proposed enhancements to those standards, where thewavelengths are 12 cm and 6 cm respectively for 2.4 GHz and 5 GHZ, it isonly necessary to retransmit from wireless communication device 102 anymessage with three similarly delayed signals to make the transmissionappear generally across PICONET 100 as badly corrupted data except atthe location of wireless communication device 110. At the location ofwireless communication device 110, the signal received by wirelesscommunication device 110 directly from wireless communication device 102will be received in phase with the signal reflections from RF reflectors111 and 112. This in-phase summation by the reflectors (environment)will allow wireless communication device 110 to extract the originalsent message.

As will be understood by persons skilled in the relevant arts given thedescription herein, the invention is not limited to the embodiment shownin FIG. 4. FIG. 4 illustrates two RF reflections and a direct signalbeing received by wireless communication device 102. In a more complexembodiment, there will be many reflections from the environment and thesignal transmitted from wireless communication device 102 to the mobilewireless communication device 110 will be a complex sum of all thesereflections. This is accomplished by a technique commonly known as atime reversal mirror where the received summed signal from wirelesscommunication device 110 at wireless communication device 102 is storedand later retrieved in reverse order for encryption of the signals sentfrom wireless communication device 102 to wireless communication device110. In many instances, the direct signal will be subtracted from thetime reversal mirror and not sent out from wireless communication device102 so that only the signals that are reflected will arrive at wirelesscommunication device 110 in phase via the RF reflectors.

FIG. 5A illustrates another example embodiment of the present invention.In this example embodiment, there are a plurality of antennas such as,for example, antennas 113 and 114 all connected to wirelesscommunication device 102. If the received signals from these antennasare summed, the location of any transmitting device in PICONET 100 willbe encoded into the summed signal. The delayed coherent signals willinclude the delays of signal travel in the coaxial or the electricalcables that connect wireless communication device 102 to the antennas.These compound delays, which can also be referred to as phasedifferences, will be reversed in the time reversal mirror andconsequently automatically accounted for in the properties of the timereversal mirror. Wireless communication device 102 can then multiply itscommunications into the time reversal mirror for transmitting over theplurality of antennas. In this manner, it is with a high degree ofprobability that the only place in PICONET 100 that will receive anuncorrupted signal (e.g., in-phase signal as illustrated by FIG. 5B) isthe original transmitter in PICONET 100 or wireless communication device110. FIG. 5B illustrates that wireless communication device 110 is in anin-phase space or location while wireless communication device 115 is inan out-of-phase location.

In some embodiments such as a shop or office, for example, the entrancecan be enhanced with reflectors and/or antennas to facilitate a log inprocedure where the mobile wireless communication device entering thefacility passes up its UUID and performs its Discovery Protocol with thefacility and the facility downloads the session key to the mobilecommunication device for that visit or attendance. Such a specialconstruction at the entrance ensures optimal isolated reception in aspecifically designed FEMTONET space. As noted herein, a FEMTONET is avolume that is a small fraction of a PICONET and in embodiments is onthe order of a one thousandth of the volume of the PICONET.

The technique described herein is designed to send via multiple pathsradio frequency energy that arrives at the FEMTONET in-phase and is thusadditive and produces a strong signal. It is, however, possible althoughunlikely that other nodes or FEMTONETs in the total irradiated volume ofthe PICONET could receive the multiple path radio frequency energyin-phase or in sufficient phase to be interpretable. These in-phaseFEMTONETs in general would be random and made even less likely to occurwith a suitably complex antenna and/or reflector arrangement.

Example Method Embodiment of the Present Invention

FIG. 6 is a flowchart illustrating the steps of a method 600 forestablishing secure short-distance communications. As illustrated inFIG. 6, method 600 includes four steps 602, 604, 606, and 608.

In step 602, communications are established between a first mobilewireless communication device and a second fixed-point wirelesscommunication device. This step is performed, for example, when awireless communication device enters within the operating range of aPICONET master communication device. Step 602 typically involves themobile wireless communication device entering into a communicationsprotocol with A facilities wireless communication device (the PICONETmaster) so that it can become a node of the PICONET. This communicationsprotocol is conducted in PICONET communications mode.

In step 604, the fixed-point wireless communication device transmits, inFEMTONET communications mode, a session encryption key to the mobilewireless communication device. As described herein, in embodiments, thefacilities wireless communications are equipped with RF reflectorsand/or antennas, and time reversal electronics. This then enables thefixed-point wireless communication device to communicate in FEMTONETmode with selected mobile wireless communication devices and supplythese devices with encryption keys for encrypted communication sessions.As described herein, in FEMTONET communications mode, only the wirelesscommunication device intended to receive a transmitted message (e.g., anencryption key) is capable of receiving the message. Wirelesscommunication devices in the PICONET that are not the intendedrecipients of the message hear only noise such as, for example,out-of-phase noise that cannot be interpreted at their spatial location.

In step 606, the mobile wireless communication device now sends, inPICONET communications mode (session key encrypted communications),identification information to the facilities or fixed-point wirelesscommunication device. In an embodiment, this identification informationincludes biological information from an individual possessing the mobilewireless communication device. This biological information can include,for example, print ridge pattern information from a finger or a thumb ofthe individual possessing the mobile wireless communication device.

In step 608, the mobile wireless communication device and thefixed-point communication device send and receive, in PICONETcommunications mode, encrypted information based on the encryption key.Thus, the transmitted information is secure even though the informationis being transmitted in PICONET communications mode. In an embodiment ofthe present invention, only the information sent by the mobile wirelesscommunication device is encrypted and the general or non-confidentialinformation sent by the fixed-point wireless communication device is notencrypted because there is no need to encrypt the information. In stillanother embodiment, the mobile wireless communication device onlyencrypts selected, confidential information transmitted to thefixed-point wireless communication device.

Example Application of the Present Invention

FIG. 7 is a diagram illustrating an example application 700 of method600. Application 700 involves a customer 704 wanting to make a secureelectronic purchase from a merchant (not shown).

Application 700 starts with customer 704 walking towards a cash register702 to purchase a selected good. Customer 704 is carrying a portablewireless communication device 706 that permits customer 704 to purchasethe selected good electronically. See, for example, commonly owned U.S.patent application Ser. No. 10/284,453, filed Oct. 31, 2002, which isincorporated herein by reference in its entirety, for a detaileddescription of how to conduct a secure electronic sales transactionusing a portable wireless device such as portable wireless communicationdevice 706.

Cash register 702 is the master of a PICONET. As the customer 704carrying the portable wireless communication device 706 approaches cashregister 702, the portable wireless communication device 706 enterswithin the PICONET of cash register 702. After the portable wirelesscommunication device 706 enters the PICONET of cash register 702, theportable wireless communication device 706 and the cash register 702engage in a discovery protocol so that portable wireless communicationdevice 706 can become a node of the PICONET.

Cash register 702 includes a plurality of antennas or is in a locationwith multiple RF reflectors (not shown) that enables it to time reversethe location of portable wireless communication device 706. In anembodiment, the locator carries out a time-reversal-mirror function todetermine the location of portable wireless communication device 706.Cash register 702 continually updates the location of portable wirelesscommunication device 706 as customer 704 moves about. In an embodiment,the locator determines an approximately spherical volume of space inwhich portable wireless communication device 706. The diameter of thisapproximately spherical volume of space is less than three feet. In oneembodiment, the diameter of this approximately spherical volume of spaceis less than one foot.

In one embodiment, after the discovery protocol is complete, customer704 is prompted by portable wireless communication device 706 to place athumb on a sensor 712 of portable wireless communication device 706.Portable wireless communication device 706 then captures customer'sthumb print ridge pattern and transmits it along with a universallyunique identification number associated with portable wirelesscommunication device 706. Other identification data can also be sent atthis time. In an embodiment, this identification information isencrypted prior to being sent, for example, in the manner describedbelow.

Upon receipt of the identification information transmitted by portablewireless communication device 706, cash register 702 switches its modeof communications from PICONET communications mode to FEMTONETcommunications mode. Cash register 702 then send an encryption key toportable wireless communication device 706. Because the encryption keyis transmitted in FEMTONET communication mode to the small sphericalvolume of space containing portable wireless communication device 706,the encryption key cannot be intercepted by other wireless devices inthe PICONET such as, for example, portable wireless communicationdevices 708 and 710. Passing the encryption key to portable wirelesscommunication device 706 in this manner allows portable wirelesscommunication device 706 to be easily and securely added to the PICONETof cash register 702.

Once portable wireless communication device 706 has received theencryption key from cash register 702, portable wireless communicationdevice 706 is able to send and receive, in PICONET communications mode,encrypted messages based on the encryption key to cash register 702. Themessage secure message can include, for example, credit card numbers,debit card number, bank account numbers, et cetera.

As will be understood by persons skilled in the relevant arts, exampleapplication 700 is not intended to limit the present invention, but isintended to illustrate the power and usefulness of the presentinvention.

For the purpose of brevity and clarity, a “locator” as used herein cancomprise a plurality of antennas and/or RF reflectors, either ofdeliberate construction or occurring naturally in the environment, andan electronic or mechanical means of performing time reversal.Electronic and mechanical means for performing time reversal will beknown to persons skilled in the relevant arts given the descriptionherein, and thus need not be described.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention as definedin the appended claims. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

1. A system for conducting sale transactions using short-range wirelesscommunications, comprising: a point-of-sale terminal that includes awireless communication device having a locator that analyzes radiofrequency signals received via a plurality of radio frequency paths todetermine a spatial location from which the radio frequency signalsoriginated; and a portable consumer wireless communication device thatcommunicates with the wireless communication device of the point-of-saleterminal using radio frequency signals, wherein the wirelesscommunication device of the point-of-sale terminal transmits a key byradio frequency signals that arrive via a plurality of radio frequencypaths substantially in-phase within a selected communication spacesurrounding the portable consumer wireless communication device, thecommunications space being centered around the spatial locationdetermined by the locator, and substantially out-of-phase at locationsoutside the selected communication space surrounding the portableconsumer wireless communication device, thereby allowing the key to bereceived by the portable consumer wireless communication device.
 2. Thesystem of claim 1, wherein the selected communication space surroundingthe portable consumer wireless communication device is an approximatelyspherical volume.
 3. The system of claim 2, wherein the approximatelyspherical volume has a diameter of less than three feet.
 4. The systemof claim 3, wherein the approximately spherical volume has a diameter ofless than one foot.
 5. The system of claim 1, wherein the locatorcarries out a time-reversal-mirror function.